1. Field of the Invention
The invention relates generally to hydraulic actuator assemblies. In one of its aspects, the invention relates to a pair of hydraulic actuators rigidly ganged together wherein the piston rods of the cylinders are extendible in parallel, longitudinally opposed directions. In another aspect, the invention relates to a hydraulic actuator assembly comprising a pair of hydraulic actuators rigidly ganged together wherein at least one of the piston rods is prevented from rotating relative to the hydraulic cylinder in which it is reciprocally mounted. In another aspect, the invention relates to a hydraulic actuator assembly comprising a pair of hydraulic actuators rigidly ganged together in which at least one of the piston rods is prevented from rotating relative to a hydraulic cylinder wherein one of the piston rods is attached to a concrete delivery chute of a concrete mixer truck, and the other piston rod is attached to the chassis of the concrete mixer truck.
2. Description of the Related Art
Hydraulic actuators are well known in the art and comprise a cylinder, a piston moving longitudinally within the cylinder, a piston rod coaxially connected to the piston and extending coaxially outside the cylinder, and end caps for fluidly sealing the cylinder. The cylinder contains hydraulic fluid under pressure, the piston rod is the moving element, the piston separates the cylinder into two separate cavities, and the end caps seal the cylinder to contain the fluid inside the cylinder. The piston is typically not restrained against rotation within the cylinder. Typically some type of mounting ends are used to affix the cylinder and piston rod in place.
Hydraulic actuators are employed in numerous applications to move a first element or part of an assembly relative to a second element. As an example, hydraulic actuators have been used to selectively position a concrete delivery chute on a front unloading concrete mixer truck. The actuator enables the chute to be readily positioned by the operator for delivery of the fresh concrete to a selected location. A typical hydraulic actuator used in this application comprises a single rod, double acting actuator. The cylinder is attached to the first part of the assembly, such as an anchoring point on the transit mixer chassis, and the piston rod is attached to the second element, such as the delivery chute. The attachments prevent rotation of the piston rod relative to the cylinder.
In order to provide an extended range beyond the range enabled by the stroke of a typical single rod actuator, a dual (or piggyback) cylinder actuator assembly was developed. The dual cylinder actuator assembly comprises two cylinders laterally connected together with the piston rods extending longitudinally in opposite directions to give approximately twice the piston rod stroke of a single rod actuator. However, because the dual cylinder actuator assembly is mounted at each end by a piston rod and not by a fixed cylinder base end, as is the case with a single rod actuator, the dual cylinder actuator assembly is suspended between the concrete delivery chute and the transit mixer chassis, which are moveable relative to each other. This relative movement will translate into rotation of the cylinders relative to the pistons and piston rods.
When the transit mixer travels down the road, the dual actuator is free to swing back and forth, which can seriously damage the actuator assembly and render the transit mixer unusable. It can also cause hydraulic connecting hoses attached to the cylinders to be pulled loose, resulting in the loss of hydraulic fluid.
To solve this problem, external anti-rotation devices have been developed to prevent the cylinders from rotating. These devices are typically added to the exterior of one of the dual cylinders.
One such device comprises a square hollow tube welded longitudinally to the side of the cylinder tube. A solid square bar adapted to slide inside the hollow tube, is inserted into the hollow tube and affixed to the mounting end of the piston rod. As the cylinders translate relative to the piston rods, the square solid bar translates along the hollow square tube (like a trombone slide), thereby preventing the piston rod from rotating relative to the cylinder. However, this assembly is very bulky and heavy, and subject to external damage. Furthermore, the assembly requires constant greasing and maintenance. The cylinder is regularly covered with cement dust which collects between the solid bar and the hollow tube, and causes them to stick together and/or travel inconsistently. Unless greasing and maintenance are performed regularly, the buildup of cement dust can harden, and the device can consequently break or no longer function properly. The device is also subject to the intrusion of water, leading to rusting which can also cause it to prematurely fail.
This device is also difficult to mount on the mixer, as the alignment between the non-rotational rod mount and the cylinder mount is critical and difficult to keep in alignment during installation. The device is also difficult to manufacture and therefore costly.